The Obama Justice Department instructed San Francisco police officials not to comment after top law enforcement officers there signed a letter accusing Weathermen radical Bill Ayers and his wife, Bernardine Dohrn, of being directly behind the 1970 bombing of San Francisco’s police station that killed one sergeant and wounded nine others.

At a press conference March 12 directed by activist Cliff Kincaid of America’s Survival Inc, the leaders of the San Francisco Police Officers’ Association made public a letter pointing a finger at Ayers and Dohrn that demands those responsible for the bombing be brought to justice.

“There are irrefutable and compelling reasons to believe that Bill Ayers and his wife Bernardine Dohrn are largely responsible for the bombing of Park Police Station,” the officers stated in the letter.

The letter called for the U.S. to bring “those responsible for the murder of Sgt. Brian McDonnell and the injuries to other officers to the justice they have so long eluded.”

Now the San Francisco Chronicle is reporting the police group members who signed the letter received calls from the Justice Department and a local police chief telling them to remain silent.

Police Officers’ Association President Gary Delagnes confirmed to the Chronicle his union was contacted by federal investigators telling them they had an “active investigation and should not be commenting on the case.”

Delagnes told the newspaper his group’s letter was meant only to show support for the family of the slain officer and to help them “bring closure to the case.”

No one has ever been charged in the bombing. Ayers has denied involvement. In a November interview with the New Yorker, Ayers said, “We killed no one and hurt no one.”

But a former FBI informant who reportedly infiltrated the Weathermen in the 1970s says Ayers described to him at length how Dohrn personally placed a pipe bomb outside the San Francisco police department Feb. 16, 1970.

The former informant, Larry Grathwohl, was present at Kincaid’s press conference earlier this month.

In his book, “Bringing Down America – An FBI Informer with the Weathermen,” Grathwohl describes a meeting in which Ayers allegedly revealed Dohrn’s role in the bombing. Granthwohl also testified to that effect to the Senate in the 1980s.

Grathwohl quotes Ayers telling him, “Too many of you are relying on your leaders to do everything.”

He said Ayers then mentioned the San Francisco bombing.

“It was a success,” Granthwolhl wrote, quoting Ayers. “But it’s a shame when someone like Bernardine has to make all the plans, make the bomb and then place it herself. She should have to do only the planning.”

Aside from the officer killed in the blast, Robert Fogarty, another officer, was severely wounded in his face and legs and was partially blinded. Eight other officers were wounded.

Speaking at Kincaid’s press conference, Jim Pera, an officer who was first to arrive after the bombing, described the scene of the attack:

“The window to the business office and an interior window, where prisoners were processed for booking were blown out. The walls and furniture were pockmarked by shrapnel. Barbed-wire fence post staples, from the bomb, were scattered throughout the ground floor of the station. Blood was all over the floor, desks and walls and was heaviest where Sergeant Brian V. McDonnell suffered mortal wounds to his neck, eyes, face and brain.”

Although no one was charged, the case is still open. Last year, it was confirmed the Justice Department in Washington, D.C., was asked to analyze fingerprint evidence from the scene of attack. The department has so far refused numerous requests to release the evidence in the case.

Ayers was the object of much attention during last year’s presidential race after it was revealed he had an extensive relationship with Obama.

On the American Thinker website, writer James Simpson commented, “Given … Bill Ayers’ perceived status as President Obama’s friend and mentor, the Justice Department may be reluctant to provide all of the evidence in the Park Station bombing case unless they are forced to do so.”